D-FISH can be performed on 1 mL of bone marrow aspirate or 5 mL of blood collected in sodium heparin. DNA probes that map to and span the common breakpoints of the ABL1 oncogene at 9q34 and the BCR at 22q11.2 are hybridized to the chromosomes of cells from bone marrow or peripheral blood. These probes are visualized by fluorescent microscopy. Normal metaphase and interphase cells have 2 red signals signifying the hybridization of the ABL1 probe to both chromosomes 9 at band 9q34, and 2 green signals signifying the hybridization of the BCR probe to both chromosomes 22 at band 22q11.2. Cells with a t(9;22)(q34;q11.2) have 1 red signal from the normal chromosome 9 at 9q34 and 1 green signal from the normal chromosome 22 at 22q11.2. In the abnormal chromosomes, parts of the hybridization sites for each of these probes have translocated to the other chromosome. Thus, 2 signals representing a fusion of red and green signals are observed (D-FISH); 1 on the abnormal chromosome 9 and the other on the abnormal chromosome 22. A small percentage of normal interphase cells will appear to have fusion signals because of the coincidental juxtaposition of 9q34 and 22q11.2.(Dewald GW, Wyatt WA, Juneau Al, et al: Highly sensitive fluorescence in situ hybridization method to detect double BCR/ABL fusion and monitor response to therapy in chronic myeloid leukemia. Blood 1998;91:3357-3365; Buno I, Wyatt WA, Zinsmeister AR, et al: A special fluorescent in situ hybridization technique to study peripheral blood and assess the effectiveness of interferon therapy in chronic myeloid leukemia. Blood 1998;92:2315-2321)